Linux Virtualization - Oct 2014 - [PATCH RFC v3 3/3] virtio-net: optimize free_old_xmit_skbs stats

From: Jason Wang <jasowang at redhat.com>

We already have counters for sent packets and sent bytes.
Use them to reduce the number of u64_stats_update_begin/end().

Take care not to bother with stats update when called
speculatively.

Based on a patch by Jason Wang.

Cc: Rusty Russell <rusty at rustcorp.com.au>
Signed-off-by: Jason Wang <jasowang at redhat.com>
Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
---
 drivers/net/virtio_net.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index b83d39d..c2b69f8 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -233,16 +233,22 @@ static unsigned int free_old_xmit_skbs(struct netdev_queue
*txq,
 	       (skb = virtqueue_get_buf(sq->vq, &len)) != NULL) {
 		pr_debug("Sent skb %p\n", skb);
 
-		u64_stats_update_begin(&stats->tx_syncp);
-		stats->tx_bytes += skb->len;
 		bytes += skb->len;
-		stats->tx_packets++;
-		u64_stats_update_end(&stats->tx_syncp);
+		packets++;
 
 		dev_kfree_skb_any(skb);
-		packets++;
 	}
 
+	/* Avoid overhead when no packets have been processed
+	 * happens when called speculatively from start_xmit. */
+	if (!packets)
+		return 0;
+
+	u64_stats_update_begin(&stats->tx_syncp);
+	stats->tx_bytes += bytes;
+	stats->tx_packets += packets;
+	u64_stats_update_end(&stats->tx_syncp);
+
 	netdev_tx_completed_queue(txq, packets, bytes);
 
 	if (sq->vq->num_free >= 2+MAX_SKB_FRAGS)
-- 
MST